The invention relates to a method and apparatus for the four-wire type transmission of digital message signals between a superordinate system, such as a digital exchange, and a subordinate system, such as a digital subscriber station, along a two-wire transmission line. The superordinate and subordinate systems repeatedly and alternately transmit to each other message signal blocks which comprise at least one message signal word, a plurality of bits serving for signalling or control and, in the transmission direction from the superordinate system to the subordinate system, a plurality of bits serving for synchronization. A message signal block is sent out by the subordinate system each time it receives a message signal block from the superordinate system, in some cases after the expiration of a prescribed time delay.
As a result of this four-wire type utilization of the two-wire line and, in particular, the successive transmission in both directions along the line, the range of such a transmission method is limited not only by the cable attenuation of the transmission line but also by the propagation time of the message signal blocks.
If the subordinate systems constitute digital subscriber stations and the superordinate systems digital exchanges, subscriber line connection lengths of up to 10 km are theoretically possible. However, depending upon the transmission frequency and type of encoding, the distance ranges attainable until now are only in the order of 5 to 6 km.
It is, therefore, an object of the present invention to provide a transmission method and apparatus for which the lengths of the subscriber lines can be in the order of magnitude of the theoretical maximum.